What if I don’t have computer to unlock my passcode

If it is Passcode Lock, you will need a computer to restore it. Perhaps you have a friend who could help. If not, make an appointment at the nearest Apple Store for assistance. Instructions here...

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Since he bought the phone online, I am a afraid you may encounter Activation Lock which requires the Apple ID and Password of the original owner who set the lock. To learn about Activation Lock, read these links....

How to remove Activation Lock - Apple Support

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Your son bought a phone which is locked and you don't know the Passcode? I'm going to guess you also don't know the Apple ID and Apple ID Password which was used to set up the phone.

Your son has pretty much been scammed and he more than likely purchased a stolen iPhone. Without the credentials used to set up the phone, there is NO WAY for your son to use the phone, UNLESS the person he purchased it from removes the phone from iCloud. Again, I'm betting the person who sold the phone dumped a stolen phone on someone unsuspecting.

If your son can't reach the person he bought the phone from, it's a lost cause. Chalk it up to a bad purchase and recycle the phone responsibly. Apple will NEVER unlock a phone without valid proof of original purchase with the phone's serial number clearly printed on the receipt. And there is no way we can help you get into a locked phone.

The links provided above would be great, if this phone was originally set up by your son and it was simply a matter of forgotten ID's/Passcode. That's not the case here though.

Read this user tip I wrote about used devices --> https://discussions.apple.com/docs/DOC-250001038